Posts Tagged ‘field corn’

Corn and Soybean Disease Update – August 12, 2011

Friday, August 12th, 2011

Bob Mulrooney, Extension Plant Pathologist;

Gray leaf spot is increasing in irrigated corn but it is too late to affect yield. This late in the season we often see an increase in gray leaf spot as well as Northern corn leaf blight.

We have seen several samples of ears with the milk line half-way up the kernels with random discolored kernels. This is Fusarium ear rot caused by Fusarium moniliforme. The fungus travels down the silks and infects the individual kernels. Often white fungal growth can be seen as well. Hybrids vary in their susceptibility and infection is favored by hot, dry weather. When severe the whole ear can be whitish. Often most of the fungus growth is limited to the tips of the ears. If grain is to be stored it is important to dry it sufficiently to prevent growth of the fungus to prohibit growth of the mycotoxin, fumonisin.

Individual random kernels and some tip infection by Fusarium moniliforme.

Despite the hot, dry weather downy mildew is showing up in full season irrigated soybeans now. Varieties vary in their resistance, but this fungus disease has never been yield limiting or damaging here. The disease produces numerous small yellow spots on the upper leaf surface and a tuft of grayish fungal growth on the corresponding lower leaf surface.

Downy mildew on the upper leaf surface of soybean caused by Peronospora manshurica.

Grayish tufts of the downy mildew fungus on the lower leaf surface.

Agronomic Crop Disease Update – July 8, 2011

Friday, July 8th, 2011

Bob Mulrooney, Extension Plant Pathologist;

Now is the time to start checking soybeans for soybean cyst nematode. Once soybeans have reached the 3rd trifoliate leave stage (roughly about 28-32 days from planting) the white or yellow female cysts can be seen on the roots. If you see irregular patches of stunted soybeans don’t presume the stunting is from drought. Digging the plants carefully may reveal SCN is present and could be the cause of the stunting. If you are seeing many cysts and stunting on resistant soybeans it is time to rotate out of that field to reduce SCN egg numbers.

White and yellow female soybean cyst nematodes on roots, 34 days after planting

Three corn samples arrived in the plant clinic this week with bacterial stalk rot. If you are irrigating from surface water sources, such as ponds or ditches, there is a risk of bacterial stalk rot. The bacterial can be in the irrigation water and get trapped in the whorl, the ear leaf sheath, and the ear shank. These places provide a place for water to sit and the bacteria can enter the stalks and cause a soft decay of leaf sheath, stalk, and ear shanks. It is foul smelling as well. It appears as random infected plants in the field and as a result it does not cause major losses. Corn is thought to be susceptible for a short period of time and the older the corn the less likely infection will occur. There is no chemical control for bacterial stalk rot. Treating irrigation water in the system with hypochlorite is an alternative solution.

Bacterial stalk rot


Manganese Deficiency on Corn as Related to Soil Organic Matter

Friday, June 24th, 2011

Richard Taylor, Extension Agronomist; and Phillip Sylvester, Kent Co., Ag Agent;

In past articles, we’ve talked about finding manganese (Mn) deficiency in small grains (this spring) and soybeans (last year) but in the past week or two we’ve found the problem in some corn fields as well. Symptoms of Mn deficiency in corn include stunting (Photo 1) and the typical interveinal chlorosis in which the veins remain green and the tissue between veins turns light yellow (Photo 2). Photo 3 and 4 show the field where the deficiency occurred with the good areas in the far distance. It should also be noted that the corn was drought stressed as well.

Photo 1. Manganese deficient corn plants (right) compared with normal corn (left)

Photo 2. Typical interveinal chlorosis caused by manganese deficiency in corn

Photo 3. Field view of Mn deficiency on corn

Photo 4. Field view of Mn deficiency with normal corn in the upper left corner near the woods

For this field, there was a significant difference in the soil organic matter content between the affected areas (2.9%) and the healthier areas (8.9%) (Table 1). Tissue testing of the affected corn plants (Table 2) showed that Mn was deficient while all the other nutrients were either in the sufficient range or higher than the sufficient range. This raised some interesting questions. The good corn soil sample actually contained a lower concentration of manganese than the bad corn soil sample and the soil water pH and percent hydrogen saturation of the soil sample from the good areas showed a greater amount of soil acidity than for the soil sample from the bad areas. The very high organic matter content of the good sample allowed corn growth at the low soil pH (4.9) and the chelating compounds available from the large amount of organic matter helped the plants obtain enough Mn for normal growth. In the lower organic matter areas, Mn availability suffered and was not overcome by the higher level of soil test Mn.

Table 1. Soil Test Results Including (*) Percent Base Saturation for Good and Bad Corn Areas

  Good Corn Bad Corn
Soil pH 1:1 4.9 6
Buffer pH 6.3 6.8
Organic Matter % 8.9 2.9
U of D P Sat Ratio 12 32
Mehlich 3 Phosphorus ppm P/FIV 54 126
K ppm 229 161
Ca ppm 1340 1480
Mg ppm 191 191
SO4-S ppm 41 31
Zn ppm 3.52 3.8
Mn ppm 2 4.4
B ppm 0.98 0.98
CEC meq/100 g soil 16.1 11
H* 45 15
K* 4 4
Ca* 41 67
Mg* 10 14
Na* 0 0

* Base saturation for each of the cations is given in percent of CEC occupied by that cation.

Table 2. Tissue Sample Results for Manganese Deficient Corn Plants

Corn Sample Sufficiency Range
N (%) 4.25  
P (%) 0.57 0.20-0.50
K (%) 4.73 1.50-3.00
Ca (%) 0.52 0.3-1.20
Mg (%) 0.29 0.15-0.50
S (%) 0.39 0.15-0.40
Mn mg/kg 16.0 25-100
Zn mg/kg 23.0 15-70
Cu mg/kg 14.9 5-25
Fe mg/kg 110 NA
B mg/kg 8.0 NA

Interpretation of values based on top of the plant.

In the final analysis, the health of the crop returns to both frequent scouting to pick up problems such as this and to emergency foliar application of manganese, either as a chelated Mn or as manganese sulfate. An application of 0.5 to 2 lbs of Mn per acre should support the corn plant until the root system has penetrated deep enough in the soil to reach the more acidic subsoil where Mn availability is likely to be greater. The exact rate depends on the size of the corn plants, the amount of coverage possible with the intended sprayer, the stress level the corn is under at application time, and the willingness of the grower to possibly make a second application if the first application is not sufficient.


Dryland Corn and Nitrogen Sidedressing

Friday, June 3rd, 2011

Richard Taylor, Extension Agronomist;

I was asked recently for thoughts on sidedressing corn during the recent heat we’ve been experiencing. I thought I would share some arcane, although still relevant, information from my soil fertility and plant nutrition class at the University of Delaware. This concerns an old proven principle called (by me at least) the Sprengel-Liebig Law of the Minimum. Essentially what P. Carl Sprengel in 1828 and J. von Liebig, also, in the 1900s stated in various papers they published was that yield will be determined by whatever factor is most limiting to the crop.

For growers who irrigate their corn, chances are that the limiting factor will be something other than water availability. It may be a disease that impacts grain fill or it may be a lack of one of the essential nutrients or even a lack of enough sunlight during the growing season. We often see the latter in years where it’s cloudy or hazy for much of the summer. With respect to nitrogen (N) sidedressing, irrigated growers can easily apply a small amount of water to incorporate the N into the soil and therefore prevent ammonia volatilization losses from the urea component of the UAN (urea-ammonium nitrate) solution.

However for dryland corn producers who may feel the need to sidedress N on their rapidly growing corn crop, what likely is the most yield limiting factor their crop faces. Is it N availability? No in my opinion, the most limiting factor will always be water availability throughout the growing season. If it isn’t the total amount of N, it will be the distribution of the rainfall/moisture during the growing season. For these producers, the decision to sidedress N will come down to the speed of corn growth versus their ability to cover all the corn acreage before the corn runs out of starter N or becomes too tall to sidedress without causing crop damage.

Within the activity of sidedressing, there are some choices to minimize N loss as ammonia volatilization such as knifing in the UAN, using anhydrous ammonia rather than UAN, or using one of the urease inhibitors. Some of the old research suggests only about a 7 to 10 percent loss in N from dribbling the UAN solution on the soil surface. Even with this amount of loss, your most limiting yield factor will still be water availability throughout the growing season.


Scout Corn Fields for Micronutrient Problems

Friday, June 3rd, 2011

Richard Taylor, Extension Agronomist;

With the recent heat, corn development has been proceeding rapidly and before the corn develops past the point where you can get into the field to treat for the most common micronutrient deficiencies we see in Delaware fields you or your consultant-scout should check your most developed corn fields. Pay particular attention to fields that have had a history of micronutrient problems in corn and small grains and in fields where the soil pH is close to neutral (pH of 7.0). Many fields in Delaware begin to show manganese (Mn) deficiency as our soil pH rises towards neutral or alkaline. Application of even a ¼ of a pound of actual Mn per acre up to 1 or 2 lbs/acre in with a post-emergence weed control spray often will restore plant vigor. Mn deficiency in corn like most micronutrients shows up as an interveinal chlorosis meaning the parallel veins remain green and the tissue in between the veins turns yellow to white. The symptoms occur first on the newest growth since the plant is unable to take Mn from older tissue (the first leaves to appear and that will die soon anyway) and move it to the newly developing leaves and ears. The fact that corn is setting its ultimate yield potential even as early as the fifth leaf stage is something we often forget. Micronutrient deficiencies during this early vegetative growth will certainly reduce yields since the active growing points, such as the developing ear, are the first to suffer from a deficiency since micronutrients are not mobile in the plant.


Postemergence Pokeweed Control

Friday, May 20th, 2011

Mark VanGessel, Extension Weed Specialist;

A few questions have come in about controlling common pokeweed postemergence in field corn. We had a trial a few years ago with tall pokeweed (sprayed in late June) and had results similar to a study contacted at Southern Illinois University. Dicamba [Banvel, Clarity, Sterling]; Distinct; NorthStar, and Callisto were the best treatments for conventional corn hybrids. Glyphosate was also effective if Roundup Ready corn was planted. Our trial did not include Lightning, but the SIU trial reported good control with Lightning with Clearfield corn. For soybeans, glyphosate is the best option. In non-Roundup Ready soybeans, Synchrony was fair (but requires STS-soybeans) or FirstRate which was only fair in the SIU trial.


Soil-Applied Herbicides to Emerged Field Corn

Friday, May 6th, 2011

Mark VanGessel, Extension Weed Specialist;

There are times (like this year) that corn has been planted and is emerged without residual herbicides being applied. Or to reduce the risk of crop injury, the residual herbicides are applied as the corn begins to emerge. Most of the soil-applied herbicides can be applied to emerged field corn. Only those products containing atrazine will provide control of weeds that have already emerged; so if there are emerged weeds, you may need an additional product to help control them. The following is a table for applying residual herbicides to emerged corn with maximum height of corn at time of application.

Herbicide Maximum Corn Height
Atrazine 12 inches
Callisto 30 inches or 8 leaf
Dual II Magnum/Cinch 12 inches
30 inches with RR corn
Outlook 12 inches
Princep do not apply to emerged corn
Prowl/Prowl H2Oa 30” or 8 collar, whichever is more restrictive
Sharpen do not apply to emerged corn
Topnotch/Harness/Degree/Breakfree 11 inches


Basis no later than 2 collars
Bicep II Magnum/Cinch ATZ 12 inches
Bullet 5 inches
Expert 12 inches with RR corn
Fultime/Keystone/Breakfree ATZ 11 inches
Guardsman Max 12 inches
Harness Xtra/Degree Xtra 11 inches
Hornet WDG 20 inches
Lexar 12 inches
Lumax 12 inches
Sequence (RR corn only) 30 inches
SureStart/TripleFlex 11 inches
Verdict do not apply to emerged corn


Protecting Corn Yield With Postemergence Programs

Friday, April 29th, 2011

Mark VanGessel, Extension Weed Specialist;

Modified from an article by Bob Hartzler, Iowa State University.

Farmers have an array of products that allow weeds to be effectively controlled postemergence. These new technologies include herbicide resistant hybrids, such as Roundup Ready and Liberty Link corn, and several newer herbicides (Callisto, Status, etc.) While postemergence herbicides (2,4-D, Banvel) have been used successfully for more than 30 years to control weeds in corn, the new products offer greater flexibility in application timing, reduced risk of crop injury, and a broader spectrum of weed control. However, an understanding of weed/corn competition is needed to use these products most efficiently.

Most summer annual weeds (giant foxtail, pigweeds, common lambsquarters, etc.) begin to emerge near the time of corn planting, but significant numbers of weeds continue to emerge into late June and July. A temptation for many farmers relying on postemergence herbicides is to delay application until the crop canopy is large enough to shade out late-emerging weeds. Delaying application of postemergence herbicides may result in cleaner fields at the end of the growing season, but this approach may have serious economic consequences. A regional project investigated the effectiveness of using only glyphosate for weed control in Roundup Ready corn (Gower et al. 2003). Glyphosate was applied at several times during the growing season based on the size of the dominant weeds in the field. A total of 35 experiments were conducted in nine states, including Delaware. Most sites had high weed densities. In these studies, weed control continually improved as applications were delayed. For example, a single application when weeds were 12” tall resulted in 95% control, whereas spraying 2” weeds resulted in only 73% control. The reduced weed control was due to weeds that emerged after application, rather than an inability of glyphosate to kill the larger weeds. Looking only at weed control would suggest that delaying herbicide applications is an effective strategy to enhance weed control.

However, corn subjected to weed competition from emergence to postemergence application began to suffer yield losses when herbicide application was made to 4” weeds. Applying the herbicide when weeds were 4” tall resulted in a 3% yield loss, and each delay approximately doubled the yield loss. The reduction in corn yields due to competition prior to the postemergence application illustrates the risk of delaying treatment in hopes of minimizing problems with late emerging weeds.

Table 1. The effect of application timing on weed control and corn yields.
Adapted from Gower et al. 2003. Weed Technol. 17:821-828.

Application timing
(Weed Size)

Weed control Corn yield loss1
(Early-season competition only)

Corn yield loss2
(Early- and late season competition )



73 0 7


83 3



90 6



93 14


12″ 95 22


1 Weeds emerging after herbicide application controlled with hand weeding.
Weeds emerging after herbicide application allowed to compete with corn.

Reference: Gower, Loux, Cardina, Harrison, Sprankle, Probst, Bauman, Bugg, Curran, Currie, Harvey, Johnson, Kells, Owen, Regehr, Slack, Spaur, Sprague, VanGessel and Young. 2003. Effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn: Results of a 2-year multistate study. Weed Technol. 17:821-828.

An efficient approach is an early application of glyphosate to protect the corn yield and in addition, include a herbicide with glyphosate that will provide residual control. Herbicides to consider include: atrazine, Callisto, Hornet, Resolve, Sandea, or Steadfast. Herbicide selection needs to be based on weeds present in the field. Be sure to consider corn height restrictions as well.


When Do Corn Yields Begin to Decline With a Late Planting Date?

Friday, April 22nd, 2011

Richard Taylor, Extension Agronomist;

With the five-day forecast for New Castle County showing a chance of light rain or showers all five days, a grower asked me when to expect corn yields to begin to decline. Some of the work done in the past with planting dates indicate that the “ideal” planting window for corn, where you can expect maximum yields, is April 25 to May 10, although those dates will vary slightly as you move from north to south in the state. Many growers have developed the habit of planting corn very early and are therefore a bit concerned that we are moving from mid-April to late April with little accomplished in the northern portions of Delaware. Southern Delaware missed some of the recent rains and growers there were able to go full out planting corn this week, while many in the northern sections of the state were again delayed with wet, cold soils.

We all hope that the weather will cooperate soon so the corn and bean crops can be planted on time; but, for those not constrained by large acreages to plant, the good news is that there are still several weeks to go in the ideal corn planting window. Once we get past mid-May, corn yield potential does begin to decline — slowly at the beginning but rapidly as we move into June.


Rates for Residual Herbicides in Corn

Thursday, April 14th, 2011

Mark VanGessel, Extension Weed Specialist;

What rate of residual herbicide to use? In soybeans, use the full labeled rate because you need 4 to 6 weeks of residual control if spraying 3 to 4 weeks prior to planting, or you need the burndown activity at these rates because the weeds have gotten large. In my opinion, there is no clear cut answer for corn. A number of issues need to be considered:

Is this a no-till or conventionally tilled field?

· If it is no till you may need the higher rates because the amount of plant residue in the field;

· How far ahead of no-till planting are you spraying (so how long do you need to have residual control); and

· If spraying ahead of no-till planting, will you be coming back with additional preemergence herbicide at planting

Are you planning on a POST application? In our work, a preemergence followed by postemergence is the most effective and most consistent approach. A total preemergence approach can work if you have light weed pressure, have a dry year that limits a second flush of weeds, do not have large seeded species (morningglory, Texas panicum, cocklebur), do not have perennial weeds (horsenettle, yellow nutsedge, bermudagrass), are able to get your preemergence herbicides incorporated with timely rainfall or irrigation, or you are just plain lucky. Otherwise, most fields will need a postemergence spray.

My point of reference for a solid, one pass herbicide program is either Lexar or Lumax with additional atrazine; or a full-rate of a premix with atrazine and grass herbicide (Bicep, Harness Xtra, etc) PLUS pendimethalin (Prowl) or rimsulfuron (i.e. Basis). Any of these programs can include simazine. And I have seen all of these programs requiring postemergence treatments for acceptable weed control. None are consistently effective for full-season control.

If you need to spray postemergence then it makes sense to use a less intensive program at planting. But keep in mind, the less intensive soil-applied herbicide approach you use, the more likely you will need to spray earlier, may need to use more than one herbicide postemergence (particularly to provide residual control), and will need to be more timely with your postemergence treatment. If you can spray your postemergence herbicide before the weeds are 3 to 4 inches tall, and you are willing (able) to spray herbicide combinations (i.e. not rely solely on glyphosate), then reducing soil-applied herbicides by up to 20 to 30%, or not including all of the tankmixes, is a sound decision.